The present invention relates to a focusing error detector for use in an optical data detecting device which records on or reproduces data from a data-recording disc such as an optical disc or an optical magnetic disc with concentric or spiral data tracks and guide tracks formed on it.
The conventional focusing error detector typically comprises, as shown in FIG. 5, a semiconductor laser 1 as a light source, a collimator lens 2 for making the pencil of rays emitted by the semiconductor laser 1, parallel an objective lens 4 which focuses the parallel pencil of rays to form a beam spot on a data-recording disc 3 and which receives the light beam reflected by the disc 3, a beam splitter 5 for redirecting the reflected light beam incident to the objective lens, an optical system 6 comprising a convergent lens 6a and a cylindrical lens 6b to make the redirected light beam an astigmatic pencil of rays and to form a beam spot, and a quadrant-division optical detector 7 containing four optical sensor blocks 7a, 7b, 7c and 7d divided by two crossing boundaries each at 45 degree to the generating line of the cylindrical lens 6b. The quadrant-division detector 7 senses the beam spot formed on it, thereby detecting the focusing error of the beam spot on the data-recording disc 3 in accordance with the configuration of the beam spot on the quadrant-division optical detector 7.
The data-recording disc 3 comprises a substrate 3a made of a light-transmitting material such as glass, concentric or spiral grooves formed on the substrate 3a to define data tracks 3b and guide tracks 3c, and a data-recording medium 3d comprising a thin film or amorphous rare earth-transition metal amorphous alloy deposited over the substrate 3a by evaporation or sputtering, thereby permitting high density recording and reproduction of data.
Detection of the focusing error of a beam spot formed on the data recording disc 3 is based on the principle as set forth below. Assuming the light quantities received by the four optical sensor blocks 7a through 7d of the quadrant-division optical detector 7 are Sa, Sb, Sc and Sd, respectively, the degree of focalization "f" of a beam spot formed on the data-recording disc 3 is calculated by the expression: (Sa+Sc)-(Sb+Sd).
Since the properly focused beam spot 12 focused properly on the quadrant-division optical detector 7 is round as shown in FIGS. 6(a) and 6(b), the light quantities Sa, Sb, Sc and Sd are equal. Accordingly, the degree of focalization "f" calculated by the expression: (Sa+Sc)-(Sb+Sd) is 0 (zero).
When the objective lens 4 is too close to the disc 3 as shown in FIG. 6(c), the beam spot 12 projected on the quadrant-division optical detector 7 is a ellipse with its major axis oriented in the direction b.sub.o and in parallel to the generating line of the cylindrical lens 6b, as shown in FIG. 6(d). In such a case, the degree of focalization "f" is a negative value.
When the objective lens 4 is too remote from the data-recording disc 3 as illustrated in FIG. 6(e), the beam spot 12 on the quadrant-division optical detector 7 is an ellipse with its major axis oriented in the direction a.sub.o and at right angle with the generating line of the cylindrical lens 9 as shown in FIG. 6(f). In such a case, the degree of focalization "f" is a positive value. If the degree of focalization "f" calculated by the expression (Sa+Sc)-(Sb+Sd) is used as a focusing error signal, it is possible to determine on the basis of the value of the focusing error signal whether the distance between the data-recording disc 3 and the objective lens 4 is proper, too short or too long.
Meanwhile, the beam spot 12 formed on the quadrant-division optical detector 7 contains a shadow 13 attributable to the diffraction by the guide tracks 3c (the shadow will be referred to as a diffraction pattern). Even when the beam spot 12 is properly focused on the quadrant-division optical detector 7 as shown in FIGS. 6(a) and 6(b), the diffraction pattern 13 may vary as illustrated in FIGS. 7(b), 7(d) and 7(f) depending upon the position of the beam spot 11 in relation to the data tracks 3b on the data-recording disc 3 as shown in FIGS. 7(a), 7(c) and 7(e).
When there is no aberration by the optical system between the light source 1 and the data-recording disc 3, the diffraction pattern 13 is symmetrical with respect to the axis corresponding to the direction at a right angle with the guide tracks 3c. Therefor, to the quadrant-division optical detector 7 is positioned in such a manner that the boundary Y--Y dividing the optical sensor blocks 7a and 7b from the optical sensor blocks 7c and 7d coincides with the axis corresponding to the direction at right angles with the guide tracks 3c, the degree of focalization "f" becomes 0, regardless of the diffraction pattern 13 shown in FIGS. 7(b), 7(d) and 7(f), as long as the beam spot 11 is properly focused on the data-recording disc 3.
According to the conventional art, however, when there is an aberration in the optical system between the light source 1 and the data-recording disc 3, displacement of the beam spot 11 in relation to the data tracks 3b of the recording disc 3 as shown in FIGS. 8(a), 8(c) or 8(e) may impair the symmetry of the diffraction pattern 13 with respect to the axis corresponding to the direction vertical to the guide tracks 3c of the recording disc 3 as shown in FIGS. 8(b), 8(d) or 8(f) (this phenomenon is called crosstalk). Consequently, if the beam spot 11 is properly focused, the degree of focalization "f" is positive for the diffraction pattern 13 shown in FIG. 8(b) or negative for the diffraction pattern 13 shown in FIG. 8(f). The degree of focalization "f" is zero only for the diffraction pattern 13 shown in FIG. 8(d).
As a result, even if the objective lens 4 is placed in the range appropriate for focalization, minor variation of the distance between the objective lens 4 and the data-recording disc 3 may cause large fluctuation of the value of the focusing error signal as shown in FIG. 9, which hinders stable focus control.